The decrease in stratospheric ozone has prompted renewed efforts in assessing the potential
damage to plant and animal life due to enhanced levels of solar Ultraviolet-B (UV-B, 280-320 nm)
radiation (Caldwell 1971, 1998; Madronich et al., 1998). The effect of UV-B enhancements on plants
includes reduction in yield and quality, alteration in species competition, decrease in photosynthetic
activity, susceptibility to disease, and changes in plant structure and pigmentation (Tevini and Teramura 1989;
Bornman 1989; Teramura and Sullivan 1991). Some species show sensitivity to present levels of UV-B radiation
while others are apparently unaffected by rather massive UV enhancements (Becwar et al., 1982). This issue
is complicated further by reports of equally large response differences among cultivars of a
species (Biggs et al., 1981; Teramura and Murali 1986). About two-thirds of some 300 species and
cultivars tested appear to be susceptible to damage from increased UV-B radiation. Crops such as soybean,
winter wheat, cotton, and corn are susceptible to damage from increased UV-B radiation. All effects of
elevated UV-B on plants should be considered in the context of other factors such as water stress,
increased atmospheric CO2, tropospheric air pollution, and temperature. The effects of UV-B on plants
have been studied mostly under growth chamber, greenhouse, while a few experiments conducted under
field conditions (Krupa, 1989). There are also few studies that have examined the joint effects of UV-B
and other stress factors on plant response. The effect of UV-B on plant growth and productivity varies
seasonally and is affected by microclimate and soil fertility. For instance, soybeans are less susceptible
to UV-B radiation under water stress or mineral deficiency, but sensitivity increases under low levels of
visible radiation (Teramura, 1983). Continued studies over many growing seasons are crucial in any UV-B
impact assessment of agricultural productivity.
Methodology for the assessment of UV-B effects on plants
In this section only a very brief discussion of methods available for studying the effects of UV-B on
plants (summarized from Krupa, 1989) is provided. The measurements and physical simulation of UV-B
radiation in the growth chamber, greenhouse or under ambient field conditions is not straightforward.
Table 1 gives a summery of methods used for the examining the effects of UV-B on
plants. The general principle in the experiments to determine the effects of UV-B on
plants involves the use of a UV source (a lamp) coupled with different types of filters to
exclude bands of UV wavelength not desired in the experiment. The intensity of UV is varied by
changing the height between the lamp source and the plant canopy. Because different biological processes
exhibit different degrees of sensitivity to different wavelengths of UV-B, a mathematical response function,
the action spectrum, must be used as a weighting factor to adjust the measured UV-B flux. The various
sources of uncertainties in calculating biologically effective UV-B flux should be
considered.
Table 1
Summery of methods used to determine the effects of UV-B on plants
Greenhouse:
UV lamps and selective wavelength filters, Westinghouse FS-40 sun lamp frames with cellulose acetate or Mylar type S filters |
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Dumpert & Knacker (1985)
Mirecki & Teramura (1984) |
Growth chamber:
UV-B lamps, simulated PAR (photosynthetic active radiation) and selective wavelength cut-off filters |
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Tevini & Iwanzik (1986) |
Field exposure:
FS-40 sun lamps coupled with Aclar, Mylar and cellulose acetate filters, Modulated fluorescent lamp system for supplementing natural UV-B
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Becwar et al. (1982)
Lydon et al. (1986)
Caldwell et al. (1983a)
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UV-B effects on plants
Agricultural scientists have responded with a series of pioneering investigations on the
effect of artificial and solar UV radiation upon plant growth and development. A great variety
of physiological and morphological
plant responses to UV radiation have been subsequently demonstrated over the past years. Most
of these experiments, however, have employed UV lamps which usually emit radiation quite unlike the
radiation present in the normal terrestrial solar spectrum. The importance
of solar angle, atmospheric turbidity, elevation above the sea level, cloud cover, total atmospheric
ozone column, and the UV albedo of the earth's surface with respect to the total UV irradiation intensity and
wavelength composition should be considered in UV radiation of natural environments. Though not all the
plant responses demonstrated as the result
of UV radiation are considered as damaging or disadvantageous for the
plant; the majority of evidence indicates that UV irradiation is usually detrimental, particularly UV-B
irradiation (Caldwell, 1971). In this section a summery of the UV-B effects on crops from the literature
will be presented. The growth of many plant species is reduced by enhanced levels of UV-B
radiation (Teramura et al., 1989). The enhanced UV-B radiation generally has negative impacts on growth,
yield and quality of some crop
plants such as soybean, winter wheat, rice, sorghum, cotton and corn. The response varies with different
plant species. Some are very sensitive and some are least sensitive. With enhanced UV-B radiation
photosynthesis decreases, plant height and leaf area decrease, dry matter production,
yield and quality reduces in many crops.
In the study conducted by Tevini et al. (1991b) plant height, leaf area, and the dry weight
of sunflower, corn, and rye seedlings were significantly reduced with enhanced UV-B radiation.
Rice is among the most important crop
plants in the world. Sixteen rice cultivars from several different geographical regions were grown in
greenhouses with supplemental levels of UV-B radiation (Teramura et al., 1991). Alterations in
biomass, morphology, and maximum photosynthesis were determined. Approximately one-third of all cultivars
tested showed a statistically significant decrease in total biomass with increased UV-B radiation.
For these sensitive cultivars, leaf area and tiller number were also significantly reduced.
Photosynthetic capacity, as determined by oxygen evolution, declined for some cultivars. In a
six year field study of a UV-sensitive soybean, Teramura et al. (1990) presented a statistically
significant 19%-25% reduction in seed yield in five of the six years under a 25% ozone reduction level.
Reference
Becwar, M.R., F.D. Morre III, and M.J. Bureke. 1982. Effects of depletion and enhancement of ultraviolet-B (280-315nm) radiation on plants grown at 3000 m elevation. J. Amer. Soc. Hort. Sci. 107:771-779.
Biggs, R.H., S.V. Kossuth, and A.H. Teramura. 1981. Response of 19 cultivars of soybeans to ultraviolet-B irradiance. Physiol. Plant. 53:19-26.
Bornman, J.F. 1989. Target sites of UV-B radiation in photosynthesis of higher plants. J. Photochem. Photobiol. B: Biol. 4:145-158.
Caldwell, M.M. 1971. Solar UV irradiation and growth and development of higher plants. p. 131-177. In A.C. Giese (ed.) Photophysiology, Volume 4.
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Teramura, A.H., M. Tevini and W. Iwanzik. 1983. Effects of ultraviolet-B irradiance on plants during mild water stress. I. Effects on diurnal stomatal resistance. Physiol Plant. 57:175-180.
Teramura, A.H., and N.S. Murali. 1986. Intraspecific differences in growth and yield of soybean exposed to ultraviolet-B radiation under greenhouse and field conditions. Environ. and Experi. Botany. 26:89-95.
Teramura, A.H., J.H.Sullivan, and J.Lydon. 1990. Effects of solar UV-B radiation on Soybean yield and seed quality: a six-year field study. Physiologia Plantarum.80: 5-11.
Teramura, A.H., and J.H. Sullivan. 1991. Potential effects of increased solar UV-B on global plant productivity. p. 625-634. In E. Riklis (ed.) Photobiology, Plenum Press, New York.
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